1. Field of the Invention
Aspects of the present invention relate to a white-emitting monomolecular compound using excited-state intramolecular proton transfer (ESIPT), an organic electroluminescent element and a laser device using the same. More particularly, aspects of the present invention relate to a white-emitting monomolecular compound using ESIPT, which are prepared by synthesizing at least two types of compounds having ESIPT characteristics in an intramolecular state, an organic electroluminescent element and a laser device using the same.
2. Description of the Related Art
In recent years, studies for developing efficient light-emitting monomolecular compounds for a next generation flat display or white illumination light source are actively in progress. In particular, since white-emitting monomolecular compounds have various advantages, including improved stability, excellent reproducibility, and ease fabrication of devices, compared to a white luminescence system using polymeric materials or various molecular compounds, development of the white-emitting monomolecular compounds has become important more and more.
White luminescence over the entire visible area ranging from about 400 to 700 nm is generally achieved by combining two or more kinds of fluorescent or phosphorescent materials having different emission ranges.
Devices emitting substantially white light by combining two or more kinds of fluorescent or phosphorescent materials are disclosed in Korean Patent Publication No. 2003-0015870 by Eastman Kodak Company, Korean Patent Publication No. 2004-0082286 by Semiconductor Energy Laboratory K.K. of Japan, and Korean Patent Publication No. 2004-0100523 by Bistorm Co., Ltd. White-emitting monomolecular compounds which have been reported to date include “White luminescence from an assembly comprising luminescent iridium and europium complexes” by Coppo P. et al, Angew. Chem. Int. Ed. 46 (12), 1806-1810 (2005), and “An organic white-light emitting fluorophore” by Yang, Y., Lowry, M., Schowalter, C M., Fakayode, S. O., J. Am. Chem. Soc. 128, 14081-14092 (2006).
Most of the above-referenced methods utilize partial energy transfer between a higher energy band gap donor and a lower energy band gap acceptor.
According to the known methods, however, color control is quite difficult to achieve simply by mixing even a small amount of an emissive dopant with a host since light emission is easily affected by a dopant having a small band gap. This problem arises from the inter-dopant Förster-type energy transfer between the higher band gap donor and the closely located lower band gap acceptor by means of spectral matching. In addition, the energy transfer characteristics are considerably affected by the concentration and state of emitting material. Thus, if the concentration and state of the emitting material vary, color purity and stability may considerably deteriorate.
For these reasons, it has been considered that development of white-light emitting compounds is quite difficult to achieve, irrespective of the concentration or state of material.
The inventors of the present invention published a paper reporting that the use of molecules having excited-state intramolecular proton transfer (ESIPT) characteristic may restrict energy transfer between different chromophores: Sehoon Kim, Jangwon Seo, Ho Kuk Jung, Jang-Joo Kim, and Soo Young Park, “White Luminescence from Polymer Thin Films Containing Excited-State Intramolecular Proton Transfer (ESIPT) Dyes”, Adv. Mater., 17, 2077-2082, (2005) In the report, the inventors disclosed that a white luminescence organic electroluminescent element could be easily fabricated when ESIPT molecules having different fluorescent colors are dispersed in a monomolecular polymer.
However, since a monomolecular white emitter is not used in the method disclosed in the paper published by the present inventors, there is a problem with reproducibility of white luminescence. To solve the problem, the present inventors conducted intensive researches and discovered that the interaction between energy acceptor and donor can be completely restricted by appropriately designing and synthesizing ESIPT molecules, thereby implementing efficient white luminescence in a monomolecular state. That is to say, the present inventors developed monomolecular compounds capable of emitting white light covering the entire visible area, irrespective of the concentration and state of the compound, by newly designing and synthesizing imidazole and oxadiazole cyclic molecules containing hydroxyphenyl and hydroxynaphthyl groups of specific structures having ESIPT characteristics, thereby completing the present invention.